This invention relates generally to phased-array antennas and, more particularly, to phased-array antenna systems that are electronically steerable in two dimensions. The advantages of inertialess scanning antenna systems for land-based and mobile radar systems are well known. It is also well known that arrays of antenna elements can be electronically steered by subjecting a transmitted or received signal to appropriate phase delays. Although the theory of such systems is well known, their complexity and high cost have severely limited their use.
One well known technique for selectively phase-shifting a microwave beam is to employ a diode grid lens of the type disclosed in a number of prior patents. For example, U.S. Pat. No. 3,708,796 to Gilbert discloses an electrically controlled dielectric panel lens for this purpose. In accordance with this technique, the microwave beam is passed through at least one dielectric panel in which is embedded a plane network of conductive leads running parallel with the direction of the electric field of the incident wave. Switches, usually in the form of diodes, are series connected in each such lead and are spaced from each other by distances less than two wavelengths, as measured in the dielectric material. It is noted in the Gilbert patent specification that the phase shift applied to the microwave beam passing through the dielectric panel varied according to the conductive states of the switches in the conductive leads. The phase shift is maximized when the microwave radiation is passed through portions of the panel in which the switches have been opened, and is minimized in portions in which the switches have been closed. This principle was also disclosed by A. Dorne et al. in an earlier U.S. patent (Pat. No. 3,276,023). Gilbert also recognized that two such lenses could be suitably oriented to deflect a beam about two orthogonal axes. The lenses are then separated by a polarization rotator, to rotate the direction of the electric field before the beam impinges on the second lens.
The design of diode grid arrays for use in antenna structures has since been further refined in other devices and patents. In particular, it is known to employ multiple parallel conductive plates as waveguides, dividing a microwave beam into multiple beam "slices," each of which will be subject to potentially different phase shifts. The conductive plates are oriented perpendicular to the direction of the conductive leads containing the switchable diodes. Typically, the diodes are arranged on grids or strips disposed between the plates. There are multiple diodes on each strip, and multiple strips are encountered by a wave propagating through the array. A typical array might provide multiple-bit phase shifting. For example, in a "three-bit" phase shifter each waveguide element of the array includes three groups of switchable diodes, each of which provides a phase shift related to that provided by its neighboring group by a factor of two. One group of diode strips might provide a phase shift of 45 degrees, an adjacent group, having more diodes, a phase shift of 90 degrees, and the next adjacent group, having still more diodes, a phase shift of 180 degrees. The three groups together can then provide a phase shift from zero to 315 degrees in increments of 45 degrees.
An important disadvantage of diode grid arrays for phase shifting microwave beams is that the number of diodes required for an array of practical size is very large. For an array of between one and two hundred phase shifters, the number of diodes will be several hundred thousand. The power dissipated by these diodes is also large and may render the structure unsuitable for some applications. In addition, there is a practical problem in wiring the diodes for independent switching operation. Although the diodes may be switched in groups, the number of wires needed to achieve a desired deflection of the beam is still in the thousands.
These numbers must be doubled if a second such array is used to provide beam deflection in an orthogonal direction, to provide scanning of the beam both in elevation and in azimuth. As a result, the use of two diode grid lens arrays for microwave beam scanning is an unacceptable approach for many radar and communications systems with a requirement for a wide-angle two-dimensional electronic scanning antenna. A system employing two diode grid lenses provides limited scanning capabilities, and is simply too complex, too heavy, too difficult to cool, too costly, and too inefficient. Accordingly, there is a real need for an alternative approach to providing a wide-angle two-dimensional scanning antenna. The present invention fulfills this need, as will be apparent from the following summary.